fix: replace deprecated(broken) nerd font icon

[qazxcdswe123]

Some codepoints conflict with other fonts (especially CJK fonts). To ensure that the icons remain in the private use area, Nerd Fonts has changed the codepoints of some icons in recent releases, for example, nf-mdi-* icons (including over 2,000 icons) are deprecated since v2.3.3 and will be removed in v3.

See https://github.com/loichyan/nerdfix

This commit replace nf-mdi with nf-md

Before:

After:

See also: https://github.com/nvim-neo-tree/neo-tree.nvim#configuration-for-nerd-fonts-v3-users

[qazxcdswe123]

Done